1. Field of the Invention
The present invention relates to a technology for amplifying light of multiple wavelengths in a wavelength division multiplexing (WDM) transmission system.
2. Description of the Related Art
A WDM optical transmission system can remarkably improve a signal transmission capacity by collectively amplifying a plurality of wavelengths using an optical amplifier. An optical amplifier most frequently used is an optical fiber amplifier doped with a rare earth element such as erbium.
Such an optical fiber doped with erbium (herein after, “erbium doped fiber (EDF)”) amplifies signal lights in a range of 1.53 micrometers (μm) to 1.6 μm with excitation light in a 0.98 μm or 1.48 μm band. Wavelength dependency (hereinafter, “tilt”) of light signals to be collectively amplified varies according to intensity of the excitation light.
FIG. 18 is a schematic for illustrating the tilt. A horizontal axis represents wavelength and a vertical axis represents power. Inclination of power is called tilt. FIG. 19 is a schematic for illustrating a relation between excitation light and EDF gain. A horizontal axis represents wavelength and an vertical axis represents EDF gain. As shown, the tilt varies according to intensity of the excitation light.
When the excitation light is increased and the EDF gain is increased, a gain on a short wavelength side with higher energy is larger than a gain on a long wavelength side. As a result, power on the short wavelength side is larger than power on the long wavelength side. Thus, the tilt occurs. Since unintended tilt can be a cause of deterioration of a transmission quality, particularly in a long-distance WDM-transmission system, it is necessary to perform gain control to make the tilt flat by an optical amplifier, to transmit all wavelengths with equivalent qualities.
FIG. 20 is a schematic of a conventional optical amplifier. An optical amplifier 200 includes EDFs (an EDF1 and an EDF2) 201 and 202 in two stages and a variable optical attenuator (VOA) 203 provided between the EDFs 201 and 202. The optical amplifier 200 performs a control to make output power constant even when input power fluctuates.
In the EDFs 201 and 202, the tilt changes when intensity of excitation light is changed and gain changes as described above. Thus, a control unit 205 performs automatic gain control (AGC) to make gains of the EDFs 201 and 202 constant. FIG. 21 is a schematic for illustrating a control for keeping tilt constant. When input power of an input to the EDF 201 changes, the control unit 205 performs automatic level control (ALC) for keeping output power of the EDF 202 constant by adjusting an attenuation amount of the VOA 203 while making a gain of the EDF 201 constant.
As shown in FIG. 20, the control unit 205 detects light intensity of WDM signals at input and output points of the optical amplifier 200 using photodiodes (PD) 211 and 214 (PD 1 and PD 4) to perform AGC. The control unit 205 detects light intensity of WDM signals at input and output points of the VOA 203 using PDs 212 and 213 (PD 2 and PD 3) and judges an attenuation amount of the VOA 203 to perform ALC (see, for example, Japanese Patent Application Laid-Open No. 2002-368698).
The optical amplifier 200 shown in FIG. 20 is an aggregate of a large number of optical components such as the EDFs 201 and 202, a dividing unit 220 that performs optical division for the PDs 211 to 214, a gain equalizing filter (not shown), an optical isolator for oscillation prevention, a WDM filter for coupling excitation light, and the VOA 203. Therefore, a gain obtained by the optical amplifier 200 is expressed as:optical amplifier gain=EDF gain−VOA loss−optical component loss  (1)
The optical component loss indicates a total loss excluding a loss (optical loss) of a part intentionally changed in the VOA 203.
The optical component loss fluctuates (in general, increases) due to aged deterioration and failure of the optical components, vibration impact at the time of transportation, a wind pressure due to forced air cooling, occurrence of a bending loss in a winding portion of an optical fiber due to a change in temperature, and the like.
However, in the conventional technology, the power is monitored by monitoring a main signal (a light signal) on a transmission path. Therefore, when changes, it is impossible to tell whether the change in a gain of the optical amplifier is caused by an optical component loss or a change in an EDF gain. In other words, only a sum of the first term and the third term in Equation 1 is detected.
The control unit 205 judges that gains of the EDFs 201 and 202 are insufficient, and executes feedback control such that an original optical amplifier gain can be secured by increasing excitation power. As a result, since the gains of the EDFs 201 and 202 are changed, unintended tilt occurs. In other words, for example, when an optical component loss increases in the optical amplifier, the control unit 205 misrecognizes the increase as a decrease in a gain, and increases the gain with the AGC function. As a result, unintended tilt occurs.
FIG. 22 is a schematic for illustrating a light level when an optical component loss occurs in the optical amplifier. When an optical component loss occurs in a point X before input of the EDF 1 (201), the PD 1 (211) on the input side does not detect the optical component loss. An amount of loss itself between the PD 2 and PD 3 (212 and 213) for controlling the VOA 203 does not change. Thus, the control unit 205 carries out a control for the VOA 203 while keeping the same attenuation amount as that before occurrence of the optical component loss. Since it is detected that output power in the PD 4 (214) on the output side is insufficient, the control unit 205 judges that a gain is insufficient with the AGC and executes adjustment to increase excitation light and return output power to a set value. In this case, as indicated by a dotted line shown in FIG. 22, in the EDFs 1 and 2 (201 and 202), inclination of a light level is steep according to an increase in a gain. As a result, the EDFs 201 and 202 operate with gains larger than a design value to cause tilt.
The conventional optical amplifier 200 operates on the premise that once the optical amplifier 200 is assembled, a loss of an optical component in the optical amplifier 200 does not change. However, when a loss of an optical component changes significantly after the assembly, a state of tilt changes and a desired optical characteristic is not obtained.
If there is a margin in power of an excitation light source and a gain enough for compensating for the increase in the loss is secured, both AGC and ALC normally operate, and, thus, abnormality due to the loss of the optical component cannot be detected. In other words, even if abnormality occurs in a WDM transmission section for transmitting a WDM signal, the abnormality is not detected in the WDM transmission section. The abnormality is detected after the WDM signal exits the WDM transmission section, for example, in a transponder device connected for each wavelength. Thus, it takes long time until the optical amplifier 200 is recovered from a failure after occurrence of the abnormality.